Authorities were searching remote islands for cyclone victims on Sunday after the most powerful storm to hit Tonga in decades visited destruction on the south Pacific archipelago, leaving at least one person dead and several injured.

Relief efforts following Saturday's storm were concentrating on the Ha'apai islands one of Tonga's three island groups between the main island of Tongatapu in the south and the Vava'u islands to the north, Tonga's director of emergencies Leveni Aho said.

Cyclone Ian hit Tonga with gusts up to 287km/h (178mph). The storm was later downgraded from the top of five-scale destructive cyclones to category four, with gusts of up to 250km/h (155mph). On Sunday the cyclone was tracking south-east away from Tonga.

Two navy patrol boats carrying tarpaulins, tents and other emergency supplies left Tongatapu to bring help to victims who were cut off in the Ha'apai islands.

Aho said authorities had been unable to make telephone contact with 23 islands, which account for most of the inhabited islands in the Ha'apai group. "The patrol boats are still out there, going from island to island to scout for information," Aho said.

Ha'apai islands are home to 8,000 people, most of whom live on the devastated islands of Lifuka, where one person died, and Foa.

Aho estimated hundreds of people on the two islands were taking shelter in church buildings that were being used as evacuation centres.

A New Zealand air force P3 Orion plane made a surveillance flight over the disaster area on Sunday, taking pictures showing the extent of the damage.

Aho said up to 70% of homes and buildings in some areas had been flattened. "There is much more damage on the ground that we anticipated before," he said.

"At this point there's only one fatality. There are some injured but that is still manageable with the existing medical facilities there."

Tongan authorities would further assess the damage before they would consider asking for international help, Aho said.

Tonga has a population of just over 100,000 people and consists of 176 islands, 36 of which are inhabited. Its economy relies on fish export, tourism and remittances from Tongan communities overseas, with about 40% of the population living in poverty.